It is important for health monitoring of many types of structures, for example bridges, ships, aircraft, oil rigs, oil storage tanks, buildings and the like to have a permanently mounted strain monitoring sensor device that is read by an associated data collection system. Due to the locations where strain measurement is useful (buried foundations, under water, inside tanks, etc.) once installed, the strain monitoring devices are often difficult to access for reading or for maintenance. Therefore, it is very useful to be able to communicate with the installed sensor device wirelessly. Further, it is very useful to eliminate the battery from the sensor device, which requires maintenance, either replacement of the battery or the replacement of the entire sensor for fully encapsulated devices.
Strain transducers for metallic structures are typically metal film resistance devices, for example, Micro Measurements strain transducer, P/N N2K-06-55020Q. Pressure transducers for measuring gas and liquid pressure are often constructed of a strain transducer attached to a metal diaphragm that bends (strains) as a result of applied pressure. These transducers typically produce a small electrical resistance change in response to the movement (strain) of the structure to which they are attached, which is often metal. The movements are small, on the order of 0.001% to 0.01%, and the resistance changes are also very small. Typical systems using these transducers have circuitry with very stable power supplies, such as batteries with added voltage regulation, and high performance microprocessors with sophisticated sensor conditioning and high resolution analog-to-digital capability.
In prior art battery-free, RFID-powered sensor tags, transducers are typically attached to the voltage output and the analog-to-digital inputs directly available on the RFID ICs. An example of an RFID IC with sensor inputs is the IDS SL900A RFID IC. The outputs and inputs available on this or other available RFID ICs do not provide the capability to drive stable bias voltages or currents into the transducer or to detect the small resistance change with adequate resolution to produce an accurate reading. Further, the RF energy beamed at the sensor tag by the RFID reader induces current and voltages on the circuit traces and components of the sensor tag which create unwanted signals (electrical noise) which makes it even more difficult to detect the small signal changes of the strain transducer.